The invention relates to improvements in methods and mechanisms for coating traveling webs such as web of paper and the like for on-machine or off-machine coating, and particularly relates to an improved mechanism wherein the coating is carefully metered and carefully smoothed without necessitating removal of a substantial amount of the coating after it has been applied to the web.
Various methods and apparatuses have been used in the art for applying coating to traveling webs such as paper, and a discussion of the background of coater developments may be found in the book "Pulp and Paper Manufacture", 2nd Edition Volume 1, pages 498 through 517 entitled "Control Secondary Fiber Structural Board Coating". Two primary types of coaters which have gained commercial acceptance are flooded-nip coaters wherein an excess of coating is applied to a traveling sheet and the excess is wiped off and smoothed by a trailing flexible blade, and such coaters can be used to apply pigmented and nonpigmented solutions in a range of coating weights from 3 to 20 pounds per 3000 square feet with a range of solids 0% to 65% at speeds of up to 4000 feet per minute and beyond.
Another type of coater which has gained commercial acceptance is an air knife coater wherein an excess of coating is applied to a surface and a jet of air directed at the oncoming coating removes a substantial amount of the coating thereby smoothing the surface underneath. Such a coater can handle a range of coat weights on the order of 0 pounds to 18 pounds per side per 3000 square feet with a range of solids of 0% to 55% at speeds of 1200 feet per minute. In practice an air knife coating apparatus includes a means to apply a non-divergent jet of air onto the oncoming web with the air emitted from a straight slit orifice of a nozzle. The jet of air trims off the uniformly thick layer of coating which has been applied to the web leaving a desired quantity of coating on the web in a layer of uniform thickness. The finished quality of any coater paper prepared with an air knife coater is dependant to a considerable extent on the quality of the paper sheet or web inasmuch as the air jet will remove coating leaving a coated layer which follows the highs and lows of the base sheet. Another problem encountered with conventional air jet coaters is that the energy level required is substantial requiring the provision of a substantial amount of air and limitation as to the speed of travel of the web. The air jet must have sufficient force to shear the overcoat away from the coating. The air jet must have sufficient mass momentum to overcome the mass momentum of the overcoat, to bring its velocity to zero and reaccelerate it in the reverse direction. Calculations show that the speed limit due to momentum exchange is proportional to the square root of the inverse of the overcoat weight. Conventionally, an air knife coater will trim an overcoat weight of coating from the web equal to the finish coat which remains on the sheet, and although this proportion may be varied, substantial limitations are imposed by the requirement that the air jet trim the substantial amount of overcoat weight in order to leave the proper thickness of coating on the web and to leave a smooth finished surface.
It is an object of the present invention to provide an improved mechanism and method for coating which attains the quality of coating normally provided by an air knife coater and yet which does not have the limitations as to speed of travel and the disadvantages of having to handle the amount of overcoat removed that are present in conventional machines.
In the best conventional presently used air knife coater, if 6 pounds of coating is applied onto the sheet, the air knife in its smoothing levelling process will remove 3 pounds and 3 pounds remains. At times when it is necessary to increase the speed of the coater, not enough coating can be removed by using the air knife.
In accordance with the principles of the present invention, the amount of coating applied to the web is initially carefully metered by a gate roll applicator, and the principal function of the air knife is to smooth the carefully metered and accurately applied layer of coating, rather than remove the coating. A small amount of coating is removed by the air knife normally in the range of 0% to 50%, but with the careful metering and levelling of the coating in the initial gate roll application, the air knife presents a superior finished smooth product without the limitations as to speed of travel and without the necessary application of energy required to remove the coating. Also, the necessity of handling an excessive amount of removed overcoat is avoided. Heretofore high speed coating has been reserved for blade coaters, but in some circumstances, a blade coater is not desirable. For example, unbleached kraft raw stock when coated with white coating formula and smoothed with a blade exhibits show-through in high spots. The darker colored raw stock shows through the white coating. With the application of the principles of the present invention, utilizing an air knife levelling means, it will be possible to apply just slightly more coating or essentially the same coating that is required and produce a smooth coating surface with no show-through by levelling with the air knife.
Other objects, advantages and features, as well as equivalent methods and structures which are intended to be covered herein, will become more apparent with the teaching of the principles of the present invention in connection with the disclosure of the preferred embodiments in the specification, claims, and drawings, in which: